In the how-come-I-never-learned-this dept, what are precision gage balls best used for? These typically come in fractional sized sets of 3; they're basically precision ball bearings.

I assume that 3 together can be used somewhat like precision parallels, but not sure of the advantages. I've also seen balls used to measure ID holes, but would be hard-pressed to see and advantage over adjustable ball-end hole gages or (in the usual case when both diameter and straightness are to be held) gage pins.

So what unique application are precision gage balls best for? Is it just a matter of being able to hold size very close for not a lot of $$$ ?

I've seen these ball gauges used for measuring OD of a tapered shaft (micrometer won't sit flat), and in combination with a block gauge you can calculate the taper accurately. I guess they can also be used as a go/nogo type of gauge for quick inspection. I remeber borrowing a set from work and my kids had a field day with them, so they also make for a very nice toy for the children and big kids like me.

Pete....This one's easy.
I use these balls frequently.
If you're making a tapered hole, (Cone shaped), and you need to check the depth/hole size,,,,,,
you use 2 balls.
One small ball inside the taper (Deep)
And one big ball at the top of the hole.
With simple math, you can figure out if the angle of the taper is correct.
Depth of small ball + height of big ball =?
By trig-ing out the answer, you instantly know the angle of your taper.
Also handy for checking Dovetails, etc.

I'd say they are in sets of 3 for checking radius grooves. Slip 3 balls of the proper radius into the "round internal o ring groove" and slip a gage pin between them to find the diameter to the inside of the balls. trig it out and you know your size. a little grease help[s for holding everything in place. Leo

San Jose, Ca. USA "That light at the end of the tunnel just might be......"

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The gage balls are furnished in sets of three for inspection set up on a surface plate. Just like a four legged chair, one leg seems to always be shorter than the other three. A three legged chair would touch down on all corners. Uneven-ness in the flat area of set up parts ( even if they are supposed to be flat) will be taken up by the 3 point contact. Ever see a 4 legged tripod ? Yup, I think you've got it.

Thanks, MetalDoc. That was along the lines of my assumption ("used like parallels" and "3 points for a plane"). In practice, it seemed to me that the work would be pretty wobbly sitting on 3 balls on top of a dead smooth surface plate -- hard for me to see the advantages over, say, spacer blocks. Is this method actually used much (maybe for things like castings with locating points or ?). With clamping is it ever rigid enough for machining??

San Jose, Ca. USA "That light at the end of the tunnel just might be......"

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Set up balls are used all time in inspection departments. Sometimes they are used on top of a post which has adjustable height and a countersunk top. These same "jacks" are used in machining for set up when machining uneven castings where a block would not bite in the ball will. yes, they are stable enough for milling. Just like any mill setup , you have to clamp with enough pressure to keep the part from moving.

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